Optic nerve oedema at high altitude occurs independent of acute mountain sickness.

Background/aims The study aims to investigate changes in the optic nerve sheath diameter (ONSD) at high altitude and to assess correlation to optic disc oedema (ODE) and acute mountain sickness (AMS). This investigation is part of the Tubingen High Altitude Ophthalmology study. Methods Fourteen volunteers ascended to 4559 m for 4 days before returning to low altitude. Ultrasonography of ONSD, quantification of optic disc parameters using a scanning laser ophthalmoscope and fluorescein angiography were performed at 341 m and at high altitude. Pearson's coefficient was used to correlate changes in ONSD with the optic disc and AMS. Assessment of AMS was performed using the Lake Louise (LL) and AMS-cerebral (AMS-C) scores of the Environmental Symptom Questionnaire-III. All volunteers were clinically monitored for heart rate (HR) and oxygen saturation (SpO 2). Results The mean ONSD at high altitude (4.6 +/- 0.3 mm, p< 0.05) was significantly increased compared with baseline (3.8 +/- 0.4 mm) and remained enlarged throughout high-altitude exposure. This change in ONSD did not correlate with AMS (AMS-C, r= 0.26, p= 0.37; LL, r= 0.21, p= 0.48) and high-altitude headache (r= 0.54, p= 0.046), or clinical parameters of SpO 2 (r= 0.11, p= 0.72) and HR (r= 0.22, p= 0.44). Increased ONSD did not correlate with altered key stereometric parameters of the optic disc describing ODE at high altitude (r< 0.1, p> 0.5). Conclusion High-altitude exposure leads to marked oedema formation of the optic nerve independent of AMS. Increased ONSD and ODE reflect hypoxia-driven oedema formation of the optic nerve at high altitude, providing important pathophysiological insight into highaltitude illness development and for future research.